The present invention pertains generally to beamformers. More particularly, the present invention pertains to narrowband beamformers. The present invention is particularly, but not exclusively, useful as a narrowband beamformer for sonar and radiofrequency applications.
A beamformer is a spatial filter that operates on an array of sensors to enhance the amplitude of a coherent wavefront relative to background noise and directional interference. One type of beamformer, the narrowband beamformer, is used to increase resolution in a given direction (mainbeam direction) over a narrow frequency range, and suppress sidelobe signals within the narrow frequency range (directional interference).
Conventional narrowband beamformers use essentially linear components and suffer from several drawbacks. To explore these drawbacks, consider the operation of a conventional narrowband beamformer. Generally, in conventional narrowband beamformers, delays are provided to beamsteer each sensor in the direction of the mainbeam. Once the sensor array is beamsteered, a signal travelling along the mainbeam path will create a plurality of sensor outputs that are all substantially in-phase. In contrast, sidelobe signals will create a plurality of sensor outputs that are out-of-phase. Conventional narrowband beamformers then separate the in-phase signals from the out-of-phase signals to isolate the mainbeam. Specifically, conventional narrowband beamformers integrate the sensor output signals to obtain the signal power as a function of phase lag. FIG. 1 shows a graph of signal power as a function of phase lag for a conventional narrowband beamformer, showing the mainbeam and sidelobes. Unfortunately, for high mainbeam resolution, lengthy signal processing times are often required to integrate the sensor outputs.
Because the use of phase lag to discriminate between mainbeam and sidelobe signals often provides insufficient resolution, several data processing techniques have been developed to increase resolution including 1) Eigenvector Techniques, 2) a Constant Modulus Algorithm, or 3) Least Squares Techniques. Unfortunately, these techniques share a common shortcoming in that strong, highly correlated multi-path components introduce a positive bias that a conventional narrowband beamformer is unable to correct. Alternatively, increased resolution can be obtained by increasing the spacing between sensor elements. However, increased space is not always available, and for systems that are prepared to utilize existing sensor arrays, retrofitting the arrays to increase sensor spacing can be costly.
In light of the above, it is an object of the present invention to provide devices suitable for the purposes of beamforming the output of an array of sensors to increase mainbeam resolution and suppress sidelobe signals. It is another object of the present invention to provide a narrowband beamformer capable of sidelobe suppression in a high clutter environment. It is yet another object of the present invention to provide a receiver that incorporates a narrowband beamformer for the purpose of reducing the distance required between sensors in the sensor array. Yet another object of the present invention is to provide a narrowband beamformer which is easy to use, relatively simple to manufacture, and comparatively cost effective.
The present invention is directed to a receiver for receiving a mainbeam signal of approximate frequency, xcexa9, in an environment containing directional interference signals that also have an approximate frequency, xcexa9. For the present invention, the receiver includes a sensor array having a plurality of sensors. Each sensor in the array preferably has a linear response. A plurality of individual adjustable delays are respectively connected to the sensor array to beamsteer the sensor array in the direction of the mainbeam. Specifically, a separate delay is connected to each sensor in the sensor array. As such, a delay output that includes all of the signals exiting the plurality of delays is established. With this cooperation of structure, a mainbeam signal that is received by the sensor array will be processed through the delays to create a plurality of signals in the delay output that are all substantially in-phase. Further, with this cooperation of structure, a sidelobe signal (i.e. a signal caused by directional interference) that reaches the sensor array will be processed through the delays to create a plurality of signals in the delay output that are substantially out-of-phase. It is to be appreciated that the spacing and configuration of the sensor array will dictate the magnitude of each time delay required to beamsteer the sensor array in the direction of the mainbeam signal.
It is an important aspect of the present invention that the receiver includes a beamformer having a plurality of nonlinear oscillators, a summer and a matched filter. For the present invention, each nonlinear oscillator is connected to a separate delay to thereby interpose each delay between one sensor and one nonlinear oscillator. Accordingly, a plurality of branches is established with each branch containing, in sequence, a sensor, a delay and a nonlinear oscillator. Within each branch, the nonlinear oscillator operates on signals received from a delay to produce a nonlinear oscillator output.
A summer is connected to each of the nonlinear oscillators to receive and sum the output from each nonlinear oscillator. This creates a summer output. The matched filter is connected to the summer to receive the summer output and extract signals from the summer output having a predetermined frequency.
Another important aspect of the present invention is that each nonlinear oscillator is coupled to at least one other nonlinear oscillator. As such, the oscillation state of each nonlinear oscillator depends on the oscillation state of at least one other nonlinear oscillator. This combination of coupled nonlinear oscillators is hereinafter referred to as the coupled oscillator array. For the present invention, the nonlinear oscillators are coupled to each other to create a signal in the summer output having a frequency of approximately, xcexa9, in response to a mainbeam signal being received by the beamsteered sensor array. Further, when coupled in this manner, the nonlinear oscillators create a spectrum of signals in the summer output that is centered about a frequency of approximately, {overscore (xcfx89)}, in response to a sidelobe signal being received by the sensor array. The frequency, {overscore (xcfx89)}, corresponds to the average of the natural frequencies for all of the nonlinear oscillators.
In operation, the sensors are first beamsteered in the direction of the mainbeam by adjusting the delays. Next, a critical phase lag, xcex94xcfx86C is selected. As explained further below, the size of the critical phase lag, xcex94xcfx86C will determine the width of the mainbeam that is extracted by the beamformer. Once the critical phase lag, xcex94xcfx86C is selected, the coupling strength between the coupled nonlinear oscillators is adjusted to configure the coupled oscillator array to have the following two characteristics. First, the coupled oscillator array is configured to synchronize in response to signals in the delay output that are in-phase. More specifically, the coupled oscillator array is configured to synchronize for signals in the delay output having a phase lag that is less than the selected critical phase lag, xcex94xcfx86C . Second, the coupled oscillator array is configured to de-synchronize for signals in the delay output that are out-of-phase (i.e. signals in the delay output having a phase lag exceeding the critical phase lag, xcex94xcfx86C ).
With the coupling strength between the coupled nonlinear oscillators adjusted to configure the coupled oscillator array as described above, the matched filter can be used to extract the mainbeam from the sidelobe beams. Specifically, as described above, receipt of a mainbeam having an approximate frequency, xcexa9, by the sensor array will create a plurality of in-phase signals of approximate frequency, xcexa9, in the delay output. Because the phase lag, xcex94xcfx86, between these signals will be less than the critical phase lag, xcex94xcfx86C , these signals will cause the coupled oscillator array to synchronize and create a signal having approximate frequency, xcexa9, in each of the nonlinear oscillator outputs. These signals created in the nonlinear oscillator outputs will be in-phase and have approximate frequency, xcexa9. Since the signals in the nonlinear oscillator outputs are in-phase, the effect of the summer will be to create a signal in the summer output having approximate frequency, xcexa9.
Additionally, as described above, receipt of a sidelobe signal having an approximate frequency, xcexa9, by the sensor array will create a plurality of out-of-phase signals having an approximate frequency, xcexa9, in the delay output. Because the phase lag, xcex94xcfx86, between these signals will exceed the critical phase lag, xcex94xcfx86C , these signals will not cause the coupled oscillator array to synchronize. Rather, out-of-phase signals in the delay output will cause the coupled oscillator array to produce signals in the nonlinear oscillator outputs that are out-of-phase. Since the signals in the nonlinear oscillator outputs are out-of-phase, the effect of the summer will be to create a spectrum of signals centered on the frequency, {overscore (xcfx89)}, in the summer output.
It is to be appreciated that the receiver will simultaneously process both mainbeam and sidelobe signals. Specifically, the mainbeam will be processed as described above producing a signal in the summer output having a frequency, xcexa9, while the sidebeam will be processed to produce a spectrum of signals in the summer output centered on the frequency, {overscore (xcfx89)}. Accordingly, the matched filter can be used to isolate the frequency, xcexa9, to thus extract the mainbeam from the sidelobe.